This invention relates to a method for electrochemically etching a silicon substrate, and more particularly to an electrochemical etching method for use in forming a diaphragm of a diaphragm type silicon pressure sensor.
Heretofore, a diaphragm of a diaphragm type silicon pressure sensor has been formed by a chemical etching, an electron discharge method, or an electrochemical etching. In the chemical etching and the electron discharge method, however, it is difficult to exactly control a thickness of the diaphragm. Therefore, it is preferable to use electrochemical etching in which the etching is automatically terminated when a preset thickness is obtained. Such electrochemical etching is disclosed in an article entitled "An Electrochemical P-N Junction Etch-Stop for the Formation of Silicon Microstructures" by T. N. Jackson et al. published in the IEEE ELECTRON DEVICE LETTERS, VOL. EDL-2, Feb., 1981, pp 44-45.
In the disclosed electrochemical etching, the etching rate is low, i.e., 1 .mu.m/min. This means that an etching time the 5-6 hours is required in order to etch a silicon substrate of 300-400 .mu.m to 10-30 .mu.m suitable for a thickness of a diaphragm. This etching time is too long in a formation process of a diaphragm of a pressure sensor.